1. Field of the Invention
The present invention relates to the design of integrated circuits, and more specifically to a method and apparatus to reduce coupling effect on reference voltages when output buffers implemented with low voltage transistors generate high voltage output signals.
2. Related Art
Output buffers are often used in integrated circuits to drive external device/load (e.g., another integrated circuit) based on data received from another source. In general, output buffers need to generate output signals to drive external devices with suitable voltage levels compatible with the specifications of external devices to ensure that the data is accurately transferred to the external devices. For example, if an external device is designed to receive input signals at +5 Volts, then output signals need to be at a maximum voltage of +5V.
Output buffers are generally implemented using transistors, which are characterized by associated maximum permissible cross terminal voltages, and such voltages are generally referred to as the voltage specification for the corresponding transistors. For example, it is generally desirable to keep the maximum cross terminal voltage of a 1.8V transistor below 1.8V, even though up to 2.4 V is often applied across the terminals of the 1.8V transistor, in which case there is some exposure to reducing the lifetime of the transistor, etc. The 2.4V limit may be referred to as allowed maximum voltage.
It is often desirable to implement output buffers generating high voltage output signals using transistors (“low voltage transistors”) of low(er) voltage specification. In general, using low voltage transistors provides benefits such as high throughput performance, reduced electrical power consumption, lower number of fabrication masks (leading to lower fabrication costs) and high density (number of integrated circuits in a unit area). High voltage output signals may be required for conformance with the specifications of the external load/device.
In one prior embodiment implementing such an output buffer, a supply voltage (equaling the high voltage of the output signals) is applied to a low voltage transistor contained in the output buffer. A reference voltage is also applied to the low voltage transistor to prevent cross terminal voltages from exceeding the allowed maximum voltage. The reference voltage is less than the allowed maximum voltage and equals half of supply voltage in the embodiment.
One problem with such high voltage output buffers is the coupling (unwanted signal fed back from output to input) of the transitions in the high voltage output signals to the reference voltages through parasitic capacitors of the low voltage transistors. Such coupling effect may cause the voltage level of the reference voltages to exceed the corresponding allowed maximum voltage, which may reduce the life time of the low voltage transistors.
In addition, the variation in the voltage level of the reference voltage resulting from the voltage coupling, may alter the transistor behavior (in particular the effective resistance of the transistor), which results in jitter in the output signals. Furthermore, the output buffer may get stuck at one output value if the coupling voltage is too high causing the transistors to be completely turned off.
What is therefore needed is a method and apparatus to reduce coupling effect on reference voltages when output buffers implemented with low voltage transistors generate high voltage output signals.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.